About 20 kilometers 12 miles thick, this giant umbrella is made up of a layer of ozone gas O3' This

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The Ozone Layer
A GIANT UMBRELLA OVER THE EARTH
About 20 kilometers (12½ miles) thick, this
giant umbrella is made up of a layer of ozone gas
(O3). This gas is found some 15 to 35 kilometers
above the Earth's surface in the upper atmosphere
or "stratosphere".
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Like a good pair of sunglasses, the ozone layer
acts like a natural filter, blocking out most of
the sun's harmful UV (ultraviolet) rays.
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Ozone...
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The ozone layer is our natural shield against UV
rays. But, there isn't a lot of ozone up there.
Ozone is continually created and destroyed.
It's a natural cycle of events that continually
creates our shield against UV rays.
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UV plays a key role in creating and destroying
ozone. Step 1 UV-C rays strike oxygen molecules
(O2), splitting them into separate oxygen
atoms. Step 2 Each oxygen atom is free to bond.
Those that bond with O2 produce ozone, or
O3. And conversely UV-B rays destroy ozone by
breaking one of its bonds and releasing an oxygen
molecule and an oxygen atom. This is the natural
cycle of our shield.
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The problem is that our actions have tipped the
balance toward too much destruction. The result
is that more harmful UV-B rays are reaching the
surface of the Earth.
Unfortunately, ground level ozone is increasing
while stratospheric ozone decreases. We cannot
move the lower ozone gas up to help the ozone
layer.
The best solution is to continue to reduce all
sources of pollution affecting our atmosphere.
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Certain human-made chemicals are the cause...
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• The main cause of ozone loss is
  chlorofluorocarbons (CFCs).
• We've used CFCs and the other ozone-depleting
  chemicals for many purposes
• CFCs - coolants in refrigerators and air
  conditioners, solvents, foams.
• Halons - fire extinguishers.
• Carbon tetrachloride - toxic chemical used as a
  dry cleaning agent, pesticide and solvent.
• Methyl chloroform - solvent in cleaners,
• degreasers and adhesives.
• HCFCs (hydrochlorofluorocarbons) - a temporary
  replacement chemical for the more dangerous CFCs.
  
• Methyl Bromide - pesticide.

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As they drift upward, CFCs eventually go through
the ozone layer. There, the sun's UV-C rays break
them apart into chlorine or bromine atoms. Then
chlorine or bromine atoms fall onto the ozone
layer. They attack ozone by stealing an oxygen
atom away and freeing it to build an O2 molecule.
CFCs
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• There is now a huge stockpile of CFCs and other
  chemicals in the atmosphere, with lifespans of
  100 years or more.
• Almost all CFCs released in the past century are
  still in the atmosphere

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By studying the ozone layer, we have a much
better understanding of the impact we've had on
this fragile part of nature.
The Right Choices at the Right Time

• Vienna Convention for the Protection of the Ozone
  Layer
• The Montreal Protocol on Substances that Deplete
  the Ozone Layer

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Natural events leading to ozone depletion.
Recent volcanic eruptions (Mexico's El Chichon in
1982 and the Philippines' Mt. Pinatubo in 1991)
have led to a decrease in the ozone layer.
Volcanic debris can remain in the upper
atmosphere for two to three years. Scientists
also believe that volcanic debris speeds up the
destruction caused by CFCs.
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Can the ozone layer heal itself? Yes. But we
must stop releasing CFCs and other chemicals into
the atmosphere. It will take decades for the
atmosphere to cleanse itself of these human-made
chemicals. The ozone layer can heal itself, but
only if we allow it to return to its natural
balance.
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The ozone layer in the Earth's upper
atmosphere screens out... Space
Dust. UV-Rays. Smog. None of the above.
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UV-Rays
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The ozone layer is created in nature by a
reaction between oxygen and... UV-A
rays. UV-B rays. UV-C rays. None of the
above.
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UV-C rays
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The ozone layer is naturally destroyed by a
reaction with... UV-A rays. UV-B
rays. UV-C rays. None of the above.
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UV-B rays
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